Integrated Moisture Index

In a mesophication context, oaks might have a competitive advantage over competitors such as red maple when long-term soil moisture is limited (Loftis and McGee 1993). With this in mind, Iverson et al. (1996; also see Iverson et al. 1997) developed the Integrated Moisture Index (IMI), a GIS tool that uses readily available input datasets (e.g., a Digital Elevation Model, DEM) to generate a single number that ranges from 0-100 that captures where soil moisture should be higher:

where solar radiation is minimized
where flow accumulation occurs (lower positions on slopes)
and in soils with higher water storage capacity

Raster map of the Integrated Moisture Index

Using updated methods kindly shared by Matt Peters (in review, 2025), I calculated the IMI and map the raw data below. For both the raster and the HUC 12 maps, lower values are hypothesized to be better for oaks.

Mean Integrated Moisture Index per HUC 12

The map below depicts mean IMI values per HUC 12 watershed as calculated using Zonal Statistics in ArcGIS Pro.

Brief Methods

Currently the methods are contained in a manuscript that is in review. Please contact Randy Swaty for more information. Once the paper is out more information will be shared below.

References

Iverson, L.R., Scott, C.T., Dale, M.E. and Prasad, A., 1996. Development of an integrated moisture index for predicting species composition. In In: Kohl, Michael; Gertner, George Z., eds. Caring for the forest: research in a changing world, statistics, mathematics and computers, proceedings of the meeting of IUFRO S4. 11-00; 1995 August 6-12; Tampere, Finland. Birmensdorf, Switzerland: Swiss Federal Institute for Forest, Snow and Landscape Research: 101-116

Iverson, L.R., Dale, M.E., Scott, C.T. and Prasad, A., 1997. A GIS-derived integrated moisture index to predict forest composition and productivity of Ohio forests (USA). Landscape Ecology, 12(5), pp.331-348.

Loftis, D.L. and McGee, C.E., 1993. Oak regeneration: serious problems, practical recommendations: symposium proceedings, Knoxville, Tennessee, September 8-10, 1992 (Vol. 84). Southeastern Forest Experimental Station.